This invention relates to a method and means of reducing the loss of heat of evaporation of liquid, typically water.
There are industrial applications where the objective is not to evaporate water but which require a tank of water exposed to atmosphere at an elevated temperature below boiling point. Such processes include heat shrinking of plastic packaging films, cooking, washing, tanning, and dyeing. In these processes it is not possible to use a sealed lid because openings allowing continual physical access to the heated fluid are required. In these applications heat loss due to unwanted evaporation is a large component of the total operating cost.
This is a particularly difficult problem in the heat shrinking of flexible plastic vacuum packages in so called shrink tunnels.
Throughout this description the term “shrink tunnel” is used for convenience. Most of the technology described also applies to other, forms of hot water shrink equipment including immersion tunnels and dip tanks.
Evaporation of water occurs in existing hot water packaging film shrink tunnels but serves no useful part of the packaging function. It is thus an unavoidable overhead cost. It tends to discourage vacuum shrink packaging in competition with non shrinkable packaging media. New technology which reduces the cost of the shrink process would thus be of great interest to packaging suppliers with an interest in supplying shrink packaging materials.
A shrink tunnel is also a special case of the much wider problem relating to the difficulty of cost effective recovery of so called low grade (i.e. low temperature) heat. Low temperature in this sense means below boiling.
Shrink packaging tunnels usually operate in air conditioned food handling areas where heat and high humidity in the working environment needs to be avoided in order to restrict growth of undesirable contaminating micro-organisms. For this reason it is necessary to minimise the escape of hot wet air from the tunnel openings at either end. These openings are fitted with flexible curtains to minimise escape of hot water vapour but to date the only method available to minimise hot vapour loss through the curtains has been to fit a vertical flue in the top of the tunnel to provide an updraft. By reducing the internal humidity level and in particular the atmospheric pressure inside the tunnel the flue minimises the escape of water vapour into the packing room through the end curtains as they open to admit or eject packages. However, by reducing the water vapour pressure inside the tunnel the flue also maximises the evaporation rate and hence the energy wastage.
Known hot water shrink tunnels can be of a spray or immersion type. The present invention is effective with both types. Water sprays increase the water surface area, encourage evaporation and therefore maximise heat loss. The heat tunnel spray is, however, required to thoroughly heat the plastic film passing through but the evaporation of water within the spray is an undesirable side effect of the operation of the tunnel due to the large water surface area promoting high evaporation.
This high rate of evaporation can also occur when a body of liquid (water) is agitated by stirring. Thus in an immersion type shrink tunnel the passage of a conveyor and product items through the water reservoir causes the water surface to be agitated so that a fresh water surface is continually being exposed thereby leading to increased evaporation.
In a standard tunnel the unwanted evaporated water vapour pressure is reduced by allowing hot water vapour (water vapour gas being lighter than air) to rise up the flue in order to minimise the flow of hot wet air out through the tunnel exit and entrance openings. Most of the heat loss is thus via the flue connected to top of the shrink tunnel. It is known that as much as 70% to over 90% of the energy produced in a standard shrink tunnel is lost up the flue. The high energy loss is thus latent heat of evaporation disappearing up the flue in the form of excess water vapour.